Deck Calculator
Work out deck boards (with the gap and a ripped last row), joists at 12, 16 or 24 in on center, deck screws and cost, plus a DCA 6 joist span check.
Boards run parallel to the house and joists run out from it. A nominal 6-inch deck board is 5.5 in wide; a nominal 4-inch board is 3.5 in.
Add prices for a cost estimate
DCA 6 lists a maximum span of 14′ 0″ for 2x10 joists (Southern pine) at 16 in on center. Joists 12′ 0″ long fit within that even if they span the full deck depth.
Table assumptions: No. 2 grade, wet service, 40 psf live load and 10 psf dead load. Span is measured between the faces of supports and excludes any overhang. This is a planning check, not a code review; your building department has the final say.
What Deck Calculator Does
This deck calculator turns a deck’s length and depth into a material list: deck boards (allowing for the gap between them and a narrower last row), joists at 12, 16 or 24 inches on center, ledger and rim board footage, and the screws that fasten each board to each joist. Add prices and it totals the cost.
It also looks up the maximum joist span in Table 2 of the American Wood Council’s DCA 6, the Prescriptive Residential Wood Deck Construction Guide, for three species groups and joist sizes from 2x6 to 2x12. That lookup is a planning check. Your local building code, beam layout, footings and inspector decide what you can build.
The layout assumed is the common attached deck: a rectangle with the decking running parallel to the house and the joists running out from it, every board crossing every joist at a right angle.
How to Use Deck Calculator
- Enter the deck length along the house and the depth out from it, in feet
- Enter the actual board width, the gap between boards and the board length you plan to buy
- Choose the joist spacing and how many screws go into each joist
- Read the boards, joists, rim and ledger footage and screw count; check the last-row width
- Pick a joist species and size to compare against the DCA 6 span table, and add prices for a cost estimate
Formula Used by Deck Calculator
Deck board rows and boards
Rows = ⌈(D + g) ÷ (w + g)⌉; Boards = ⌈rows × ⌈L ÷ b⌉ × (1 + waste)⌉
- D
- Deck depth out from the house, in inches
- L
- Deck length along the house, in feet
- w
- Actual board width, in inches (5.5 for a nominal 6-inch board)
- g
- Gap between boards, in inches
- b
- Board length, in feet
Worked example
A 16 × 12 ft deck, 5.5-inch boards, 1/8-inch gaps, 16 ft boards, 10% waste.
- (144 + 0.125) ÷ (5.5 + 0.125) = 25.62, round up to 26 rows
- Last row: 144 − 25 × 5.625 = 3.375 inches, ripped to fit
- ⌈16 ÷ 16⌉ = 1 board per row, so 26 boards (416 linear ft)
- 26 × 1.10 = 28.6, round up
Result: 29 deck boards, 16 ft long.
Joists
Joists = ⌈L ÷ s⌉ + 1, each D long
- s
- Joist spacing on center, in inches, with L converted to inches
Worked example
The same 16 × 12 ft deck at 16 inches on center.
- 192 ÷ 16 = 12 spaces
- 12 + 1 = 13 joists, each 12 ft
- Ledger plus rim: 2 × 16 = 32 linear ft
Result: 13 joists (156 linear ft) and 32 ft of ledger and rim board.
Deck screws
Screws = rows × joists × screws per crossing
Worked example
26 rows, 13 joists, 2 screws where each board crosses each joist (the DCA 6 minimum).
- 26 × 13 = 338 crossings
- 338 × 2 = 676 screws
- 676 ÷ 192 sq ft = 3.52 per sq ft
Result: 676 screws, before spares and before fastening the rim and ledger.
Maximum Joist Spans (AWC DCA 6, Table 2)
Allowable span between the faces of supports, not counting overhang. The table assumes a 40 psf live load, 10 psf dead load, No. 2 grade lumber, wet service and L/360 deflection. The 18 ft entries are a prescriptive cap DCA 6 sets for footing design.
| Species group | Size | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|---|
| Southern pine | 2x6 | 9′ 11″ | 9′ 0″ | 7′ 7″ |
| Southern pine | 2x8 | 13′ 1″ | 11′ 10″ | 9′ 8″ |
| Southern pine | 2x10 | 16′ 2″ | 14′ 0″ | 11′ 5″ |
| Southern pine | 2x12 | 18′ 0″ | 16′ 6″ | 13′ 6″ |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2x6 | 9′ 6″ | 8′ 4″ | 6′ 10″ |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2x8 | 12′ 6″ | 11′ 1″ | 9′ 1″ |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2x10 | 15′ 8″ | 13′ 7″ | 11′ 1″ |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2x12 | 18′ 0″ | 15′ 9″ | 12′ 10″ |
| Redwood, western cedars, ponderosa or red pine | 2x6 | 8′ 10″ | 8′ 0″ | 6′ 10″ |
| Redwood, western cedars, ponderosa or red pine | 2x8 | 11′ 8″ | 10′ 7″ | 8′ 8″ |
| Redwood, western cedars, ponderosa or red pine | 2x10 | 14′ 11″ | 13′ 0″ | 10′ 7″ |
| Redwood, western cedars, ponderosa or red pine | 2x12 | 17′ 5″ | 15′ 1″ | 12′ 4″ |
Joists Needed by Deck Length
Including both end joists.
| Deck length | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| 8 ft | 9 | 7 | 5 |
| 10 ft | 11 | 9 | 6 |
| 12 ft | 13 | 10 | 7 |
| 16 ft | 17 | 13 | 9 |
| 20 ft | 21 | 16 | 11 |
Rows of 5.5-Inch Boards with 1/8-Inch Gaps
The last column is the width left for the final row.
| Deck depth | Rows | Last row width |
|---|---|---|
| 8 ft | 18 | 0.375 in |
| 10 ft | 22 | 1.875 in |
| 12 ft | 26 | 3.375 in |
| 14 ft | 30 | 4.875 in |
| 16 ft | 35 | 0.750 in |
How to Read Your Result
Avoiding a sliver at the edge
Boards rarely divide evenly into a deck’s depth. At 16 ft deep, 5.5-inch boards with 1/8-inch gaps leave a final row just 0.75 inch wide, too thin to fasten well. Ripping a little off both the first and last rows, or adjusting the gap slightly, spreads the difference. The calculator flags any last row under half a board wide.
Board length and joints
Where a row needs more than one board, the joint has to land on a joist, and DCA 6 says each piece of decking must bear on at least 3 joists. At 16 inches on center, 8, 12, 16 and 20 ft boards are a whole number of joist spaces, while 10 and 14 ft boards are not and will be trimmed back to the nearest joist.
Choosing joist spacing
Closer joists can each span farther: a 2x10 southern pine joist goes from 11 ft 5 in at 24 inches on center to 16 ft 2 in at 12 inches. The decking has its own limit too. DCA 6 assumes 2-inch nominal lumber or span-rated decking, and composite boards carry the maker’s own maximum spacing.
Fasteners
DCA 6 calls for two 8d threaded nails or two #8 screws where each board crosses each joist, with boards spaced about 1/8 inch apart. Hidden-fastener systems use clips instead, so enter the count their instructions give per joist.
Limitations & Accuracy Notes
- Rectangular, single-level decks only, with boards at right angles to the joists. DCA 6 allows decking at up to 45 degrees to the joists; diagonal or picture-frame layouts need more boards and more cutting than this estimate.
- Posts, beams, footings, joist hangers, ledger fasteners, flashing, railings and stairs are not included.
- The span check compares joist length with DCA 6 Table 2 under that table’s load and grade assumptions. It is not a code review. DCA 6 does not cover decks carrying large concentrated loads such as hot tubs, or snow loads over 40 psf, and local amendments can differ.
- The waste allowance is a flat percentage. Crooked boards, defects and tricky cuts can need more.
- Screw counts cover decking to joists only. Rim boards, fascia and stairs take additional fasteners.
Frequently Asked Questions
How many deck boards do I need?
Divide the deck depth plus one gap by the board width plus one gap, and round up to get the number of rows. A 16 × 12 ft deck with 5.5-inch boards and 1/8-inch gaps needs 144.125 ÷ 5.625 = 25.6, so 26 rows. With 16 ft boards that is 26 boards, or 29 with 10% extra for waste.
How far apart should deck joists be?
The AWC DCA 6 deck guide gives joist spans at 12, 16 and 24 inches on center, and closer spacing lets each joist span farther. A 2x8 southern pine joist can span 13 ft 1 in at 12 inches, 11 ft 10 in at 16 inches and 9 ft 8 in at 24 inches. Your decking also has a maximum span, so check the board maker’s rating before using 24 inches.
How many screws do I need for a deck?
DCA 6 calls for two #8 screws (or two 8d threaded nails) where each board crosses each joist. Multiply rows by joists by 2. A 16 × 12 ft deck with 26 rows and 13 joists at 16 inches on center has 338 crossings, so 676 screws, about 3.5 per square foot.
How much gap should be between deck boards?
The AWC DCA 6 guide says to space wood decking boards approximately 1/8 inch apart. Composite and PVC boards follow the manufacturer’s installation instructions, which can set a different gap, so read those before you start.
How far can a 2x8 deck joist span?
At 16 inches on center, DCA 6 Table 2 allows 11 ft 10 in for southern pine, 11 ft 1 in for Douglas fir-larch, hem-fir and spruce-pine-fir, and 10 ft 7 in for redwood, western cedars, ponderosa and red pine. The table assumes No. 2 grade lumber, wet service, a 40 psf live load and a 10 psf dead load.
How many joists do I need for a 16-foot deck?
Divide the length in inches by the spacing and add one for the end joist. 192 ÷ 16 = 12 spaces, so 13 joists at 16 inches on center. At 12 inches it is 17 joists, and at 24 inches it is 9.
What size is a 2x6 deck board really?
A dry, dressed 2x6 is 1-1/2 by 5-1/2 inches under the American Softwood Lumber Standard (PS 20). Nominal 4-inch boards are 3-1/2 inches wide. Enter the actual width you measure, since composite boards and imported lumber can differ.
References & Further Reading
- American Wood Council — DCA 6, Prescriptive Residential Wood Deck Construction Guide (2015 IRC edition) — Table 2 joist spans; two #8 screws per joist; 1/8-inch board spacing; 3-joist minimum bearing; scope limits
- Voluntary Product Standard PS 20-20, American Softwood Lumber Standard — Table 3: a nominal 2x6 dresses to 1-1/2 × 5-1/2 inches dry